This invention relates to a method for modifying surfaces of metallic articles, such as medical devices. This invention further relates to treatment of implant surfaces to achieve desirable rough and microporous surface features which enhances bone ingrowth and thus establishes a strong mechanical bond with the implant.
Titanium, either pure or as an alloy with a few percent aluminum and vanadium, is used as the metal for bone implantation in knee and hip joints. Titanium is commonly used because of its mechanical properties and also its bio-compatibility. However, titanium does not form a direct chemical bond with bone, which can sometimes cause loosening and failure of the implant. Rough and porous implant surfaces permit bone and bone cement to penetrate the surface pores and ideally provide a strong mechanical bond with the implant.
Irregular and rough implant surfaces with certain characteristic features can effectively promote bone ingrowth and directly bond with bone, thus enhancing long term implant stability. European Patent No. 038 8576 reports that it is desirable to have a surface macro-roughness with pore sizes of about 10-20 xcexcm with a micro roughness superimposed thereon with a pore size of less than 2 xcexcm. Thus, an implant surface with macro-roughness and micro-porosity is highly desirable and can effectively promote bone ingrowth and fixation. This dual feature surface can also be useful when bone cement material is used to adhere implants to bone (i.e., polymethyl(methacrylate)).
Achieving the desired balance of macro- and micro-porosity has not been easy. Presently, modification of implant surfaces can be classified into two distinct groups: (a) surface modification by application of a coating of rough and/or porous material; and (b) surface modification by bulk implant surface treatment.
Porous coatings disclosed in the prior art include diffusion bonded metal fiber coatings produced from titanium wire in the form of random, rough metal fibers. Pressure sintering of spherical titanium metal powders or beads on implant surfaces also has been used to produce macro-porosity (U.S. Pat. No. 4,644,942). Thermal plasma spray processes have also been used to deposit porous coatings (U.S. Pat. No. 4,542,539). Commercially pure titanium or titanium alloy powders are partially melted in the plasma flame and the molten particles impact the implant surface at high speeds. These powders rapidly quench on the surface and adhere to the implant metal thus yielding a rough surface. Another porous coating reported in the prior art is a perforated metal foil applied to a solid metal core (U.S. Pat. No. 3,905,777).
In all of the above prior art devices, an interface between the metallic core and the porous surface results. In order to be used as a implant device, the interface between the coating and the implant substrate must be strong and stable. Interfacial failure during implant insertion or after prolonged use of the implant can result in loose metal particles which can become a source of contamination in the adjacent tissue and may also cause the failure of the implant.
Yet another method of imparting surface roughness to a metallic implant device is to treat directly the surface of the solid metallic core. U.S. Pat. No. 4,865,603 discloses a method of surface treatment in which the outer surface is subjected to serial machining processes which results in a complex surface topology. A laser beam of selected power and pulse duration has been used to drill cavities on an implant surface. (U.S. Pat. No. 5,246,530). Precise positioning equipment is necessary to process the implant in both of these methods. An additional major disadvantage of the laser process is that it only provides macro cavities for bony ingrowth. Finally, EP 388 576 discloses a method of aqueous acid treatment, which has been used to achieve a rough and porous surface on implants for bone ingrowth. However, titanium hydride and other undesirable surface artifacts are formed as a result of the acid-metal reaction.
Thus, there remains a need to provide effective surface modification techniques which provide a surface suitable for bone ingrowth, which possesses strength and structural integrity and is free of surface contamination.
Plasma treatment of titanium-containing surfaces is known and used in the semiconductor industry, primarily for the etch removal of titanium-tungsten layers in semiconducting devices. See, U.S. Pat. Nos. 5,164,331 to Lin et al. and 4,203,800 to Kitcher et al. The references are directed to removal of TiW alloy to expose the underlying layers in a semiconductor device. The material is desirably removed at a constant, even rate across the entire exposed surface. Further, the resultant surface is desirably smooth and even. These and other similar prior art methods do not contemplate a process for surface treating a bulk titanium work piece so as merely to alter the surface characteristics or in particular to provide a rough, porous surface.
It is an object of the present invention to provide an article and method of manufacture of an article having a porous surface suitable for use as an implant device.
It is an object of the present invention to provide a porous surface on an implant device with high structural integrity.
It is a further object of the present invention to provide a porous surface on an implant device with high surface purity and without the formation of surface contaminants.
It is yet a further object of the present invention to provide an article and method of manufacturing an article which has a porous surface exhibiting macro- and micro-roughness.
The present invention uses a process for modifying the implant surface to form a porous surface with characteristic macroroughness and microporosity, which is close to naturally the surface structure of occurring bone tissue. The method of the invention includes exposing a titanium surface to a plasma comprising a reactive plasma gas including an active etching species and a sputtering ion. Predetermined plasma conditions are used to modify the titanium surface and provide surface porosity.
The present invention also includes exposing a titanium surface to a plasma comprising a reactive plasma gas including an active etching species and a sputtering gas. Predetermined plasma conditions are used to effect non-uniform etching and non-uniform sputtering of the titanium surface. The titanium may be commercially pure titanium or a titanium alloy.
By xe2x80x9cnon-uniform etch ratexe2x80x9d and xe2x80x9cnon-uniform sputter ratexe2x80x9d, as those terms are used herein, it is meant that the etch and/or sputter rate are not constant or even within a given cross-sectional area of the work piece surface (geographic non-uniformity). It may additionally include rates which are variable over time (temporal non-uniformity).
By xe2x80x9creactive plasma gasxe2x80x9d, as that term is used herein, it is meant to include both a gas mixture which is introduced into the plasma chamber and the plasma gases which result therefrom. Thus, the reactive plasma gas includes an active etching species and the halide gases from which it is formed; the reactive plasma gas likewise refers to a sputtering gas introduced into the plasma and the bombarding ions generated therefrom in the plasma.
In a preferred embodiment, plasma conditions are effective to redeposit sputtered species onto the titanium surface. In another preferred embodiment, plasma conditions are effective to sputter off oxygen adsorbed on the titanium surface during exposure of the surface to the plasma. In yet another preferred embodiment, a sputtering target is introduced into the plasma, said sputtering target comprising a masking element, such that the masking element is deposited onto the titanium surface during exposure of the surface to the reactive plasma gas. The plasma may be effective to take advantage of masking properties of alloyed elements in the work piece or of variable etch rates among surface deposited species on the work piece.
The present invention provides a rough and/or microporous implant surface without any contamination or deleterious effect on the bulk properties of the implant. An advantage of this invention is that the rough and porous surface is integral to the implant metal and no chemical byproducts or artifacts are formed by this method. The article comprises a metallic substrate; and a surface comprising metallic filamentous elements having a length of about one to fifteen micrometers, the filamentous elements integral with the substrate at a first end and extending in a substantially outward direction from the substrate. The article may be characterized by the filamentous elements characterized by a fused appearance or by filamentous elements characterized by an arched configuration in which the filamentous elements appear fused at a point distal from the substrate.
In one embodiment of the invention, filamentous elements are located at the substrate with a density of about 1-40 filamentous elements/xcexcm2. In another embodiment of the invention, the arched filamentous elements are located at the substrate with an arch density of about 0.01-10 arches/xcexcm2. In yet another embodiment of the invention, filamentous elements are located and arranged at the substrate so as to have the appearance of a ridge or wall. The ridge or wall may be separated from an adjacent ridge or wall by a distance of about 0.5 xcexcm to about 2 xcexcm.
By a xe2x80x9cporousxe2x80x9d as that term is used herein, it is meant a surface which is not a flat or dense surface and which exhibits a complex surface morphology. It does not require the presence of actual xe2x80x9cporesxe2x80x9d in the conventional sense.
By xe2x80x9cfilamentousxe2x80x9d as that term is used herein it is meant an elongated feature extending in a substantially outward direction from the substrate surface, having a round, flattened or tape-like appearance. The individual filamentous features may be free standing or they may be fused or otherwise agglomerated in the final surface structure. Fused or agglomerated filamentous substructures provide the surface morphology of the surface of the inventive devices.
The roughened surface and microporosity on the surface yield an increase in the surface area. Such an increased surface area may also provide an ideal substrate for chemically bioactive coatings (for example, hydroxyapatite), thereby improving adhesion of the coating to the implant surface. Further, the uneven surface morphology, with its undercuts and deeply etched gaps, are a source of physical interaction with host site to secure a medical implant and promote osseous ingrowth.
The method of the invention may be readily adapted to non-titanium surfaces, by control of the processing variables identified in the present invention.